This invention relates to MIM (Metal-Insulator-Metal) devices and their fabrication. MIM devices generally comprise on a substrate a thin film insulative layer sandwiched between two conductive layers across which, in use, a voltage is applied, the device exhibiting a non-linear resistive characteristic in operation. The invention relates also to a display device incorporating MIM devices.
MIM devices, which can be regarded as a type of diode structure, have been used in active matrix addressed liquid crystal display devices as switching elements for controlling operation of the device's picture elements. These two-terminal, non-linear devices offer the advantage over TFTs (thin film transistors) also used for such purposes in that they are comparatively simple to fabricate.
A typical MIM addressed LCD device consists of a pair of glass substrates carrying respectively a set of row address conductors and a set of column address conductors with individual picture elements being defined at the region of the intersections of the crossing row and column conductors. Each individual picture element comprises a picture element electrode carried on the one substrate with the row conductors, an opposing portion of one of the column conductors on the other substrate, together with the liquid crystal material therebetween, and is connected electrically in series with at least one MIM device between a respective row conductor and column conductor.
The MIM devices act as bidirectional switches to control operation of their associated picture elements. By virtue of their non-linear resistance behaviour, the devices exhibit threshold characteristic and in effect turn on in response to a sufficiently high applied field to allow video data signal voltages to be transferred to the picture elements to cause the desired display response. The switching behaviour of the MIM device results from tunnelling or hopping of carriers in the thin film insulative layer, and in this respect the voltage/resistance characteristic of the device is dependent on the magnitude of the electric field and thus on the nature and thickness of the insulative layer. When considering insulative layer thicknesses in the region of a few tens of nanometers the predominant mechanism in this behaviour appears to be the Poole Frenkel effect. Devices using such thicknesses of insulative layer have been found to offer more satisfactory performance for liquid crystal display device applications through their ability to provide the necessary on/off ratio at acceptable voltages.
One possible method of addressing the display device is to apply scanning voltage signals to the row conductors and data voltage signals to the column conductors. The matrix array of picture elements are addressed a row at a time to build up a display picture over one field.
In a known type of MIM structure, used in an LC display device, as described for example in U.S. Pat. No. 4,413,883, the insulative layer is formed as an anodised oxide surface layer on a metal layer constituting of the conductive layers. The one conductive layer consists e.g. of tantalum, which is anodised to form a thin film of insulative tantalum pentoxide on the surface. The insulative layer is then covered by a conductive layer of nickel, chromium, tantalum, aluminium or other metal. Anodic oxidation is a reasonably convenient process and the thickness of the oxide layer obtained can be controlled by the voltage applied for oxidation.
It is important to the successful operation of MIM devices that they have good insulation properties under low applied field conditions, so as to provide a high resistance, and that they become conductive at higher applied fields in a controlled manner to achieve characteristics similar to a forward biased diode. They need to have, therefore, appropriate non-linear characteristics suited to the operational criteria of a liquid crystal display device. These characteristics are dependent on the thickness of the insulative layer, as mentioned earlier, and are determined by the charge transfer mechanisms involved. In the above described type of structure, these mechanisms are influenced by the inclusion of impurities or defects in the anodised metal layer.
In other known types of MIM structures suitable for use in LC display devices, separately deposited thin film insulative layers are used. Insulative materials such as silicon nitride, silicon dioxide, silicon oxynitride, silicon monoxide and zinc oxide have been proposed. An example of a MIM structure employing silicon oxynitride material as the insulative layer is described in British Patent Application No. 8729517.
Although generally referred to as a Metal-Insulator-Metal device, conductive non-metal materials such as indium tin oxide (ITO) can be used as one or both of the "metal" layers.
MIM devices using insulative materials such as silicon oxynitride or silicon nitride are considered to offer superior performance characteristics in, particularly when used in a display device, by virtue of the lower dielectric constant of these materials compared with anodised tantalum, for example.
However, one problem likely to be experienced with such MIM devices is that defects such as short circuit can occur. With an array of MIM devices, as in a display device, one such short circuit can lead to an entire row or the entire array being unusable.